The invention relates to a process for the preparation of hydrocarbons starting from hydrocarbons having a smaller number of carbon atoms per molecule.
Hydrocarbons of at most four carbon atoms per molecule (hereinafter referred to as "C.sub.4.sup.- hydrocarbons") can be converted into hydrocarbons having at least five carbon atoms per molecule (hereinafter referred to as "C.sub.5.sup.+ hydrocarbons") by a two-step process in which in the first step the C.sub.4.sup.- hydrocarbons are converted by steam reforming into a mixture of carbon monoxide and hydrogen, which mixture is contacted in the second step at elevated temperature and pressure with a catalyst and thus converted into a mixture of hydrocarbons consisting substantially of C.sub.5.sup.+ hydrocarbons. The reaction which takes place in the second step of the process is known in the literature as the Fischer-Tropsch hydrocarbon synthesis. Catalysts often used for this reaction contain one or more metals from the iron group together with one or more promotors and a carrier material.
In order to increase the yield of C.sub.5.sup.+ hydrocarbons unconverted hydrogen and carbon monoxide present in the reaction product of the second step can be recycled. In order to increase the selectivity towards C.sub.5.sup.+ hydrocarbons the C.sub.4.sup.- hydrocarbons formed as by-product can also be recycled. The two-step process using recycling can be carried out by dividing the reaction product of the second step into a gaseous fraction consisting substantially of C.sub.4.sup.- hydrocarbons and unconverted hydrogen and carbon monoxide, and a liquid fraction consisting substantially of C.sub.5.sup.+ hydrocarbons and water formed during the hydrocarbon synthesis, and recycling the gaseous fraction to the first step.
Since the steam reforming of C.sub.4.sup.- hydrocarbons leads to the formation of a H.sub.2 /CO mixture having a H.sub.2 /CO molar ratio higher than 2, whilst Fischer-Tropsch catalysts have a H.sub.2 /CO consumption ratio of at most about 2, when carrying out the two-step process with the use of recycle, the excess hydrogen formed will have to be removed during the process in order to prevent H.sub.2 build-up in the system. The quantity of hydrogen to be removed is determined, inter alia, by the H/C atomic ratio of the feed for the first step and the CO-shift activity of the catalyst used in the second step. On the assumption of a stoichiometric conversion of the feed during the steam reforming, according to the equation EQU C.sub.n H.sub.2n+2 +n H.sub.2 O.fwdarw.n CO+(2n+1)H.sub.2,
the synthesis gas obtained will have a higher H.sub.2 /CO molar ratio according as the feed for the first step has a higher H/C atomic ratio, and therefore more hydrogen will have to be removed during the process. For instance, starting from methane (n=1) as feed for the steam reforming, a synthesis gas can be obtained by the reaction given above which has a H.sub.2 /CO molar ratio of (2n+1)/n=3. According as the catalyst used in the second step has higher CO-shift activity, a larger portion of the quantity of CO present in the synthesis gas will react with the water formed as by-product in the hydrocarbon synthesis according to the equation CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2, so that the H.sub.2 /CO molar ratio will increase, and therefore more hydrogen will have to be removed in the process.
In order to keep the quantity of hydrogen to be removed as small as possible when carrying out the two-step process with use of recycle starting from a feed with a given H/C atomic ratio, preference is given to the use in the second step of a catalyst with the highest possible H.sub.2 /CO consumption ratio.
In the two-step process both the product of the first step and the product of the second step contain steam. The steam present in the product of the first step has found its way into that product at least partly on account of the fact that the steam reforming reaction is an incomplete reaction, so that even when a stoichiometric quantity of steam is used, a minor quantity thereof is found in the reaction product. For the protection of the catalyst the steam reforming is usually carried out in the presence of a considerable excess of steam. When excess steam is used in the steam reforming, this excess is found in the reaction product of the first step, together with the minor amount of steam mentioned hereinbefore. The steam present in the product of the second step has found its way into that product on account of the formation of steam as by-product in the hydrocarbon synthesis in the second step according to the equation CO+2H.sub.2 .fwdarw.--(CH.sub.2)--+H.sub.2 O.
As is seen from the above, when the steam reforming reaction and the hydrocarbon synthesis reaction proceed stoichiometrically, the quantity of steam used in the steam reforming will correspond substantially with the quantity of steam formed in the hydrocarbon synthesis. In order to keep the quantity of water which must be added to the process from outside as small as possible, it is preferred that both the water which has remained unconverted in the steam reforming and the water which has formed in the hydrocarbon synthesis are used in the first step of the process. The water which is present in the reaction product of the first step in the form of steam can be removed therefrom by condensation. The water which is present in the reaction product of the second step in the form of steam is found together with the C.sub.5.sup.+ hydrocarbons in the liquid fraction obtained in the gas/liquid separation carried out after the second step. Both water streams can be recycled to the first step.
Although the above-described two-step process, in which not only unconverted hydrogen and carbon monoxide and C.sub.4.sup.- hydrocarbons formed as by-product are recycled to the first step, but also water from the reaction products both of the first and of the second step, offers the possibility of highly selectively preparing C.sub.5.sup.+ hydrocarbons, whilst the quantity of water which has to be fed to the process from outside is kept as small as possible, this process has a severe drawback. This drawback concerns the way in which the water is separated from the reaction products of the first and second step. As stated hereinbefore, this separation is carried out by condensation. This involves that steam whose pressure was originally at the process level be separated in the form of water, from which subsequently steam must be formed which must be re-pressurized to the process level before it can be introduced into the steam reforming. In view of the often big excess of steam used in the steam reforming and the considerable amount of steam formed in the hydrocarbon synthesis (owing to the development of the Fischer-Tropsch reaction the reaction product of the second step contains more water than hydrocarbons, expressed by weight), this procedure entails high cost when carried out on a technical scale.
Naturally it would be much more attractive to leave the steam which has remained unconverted during the steam reforming in the reaction product and not to separate it until after the second step, together with steam formed in the second step. By dividing the reaction product of the second step into a liquid fraction consisting substantially of relatively high-boiling hydrocarbons and a gaseous fraction consisting substantially of unconverted hydrogen and carbon monoxide, stream and relatively low boiling hydrocarbons, and recycling the gaseous fraction to the steam reforming, a steam recycle might be brought about without there being the need--in order to separate steam--of successive condensation, evaporation of the water and re-pressurizing of the steam. However, application of this process on a technical scale is to a considerable extent dependent on the influence which steam has on the behavior of the catalyst in the second step and the selectivity of this catalyst to the formation of relatively high-boiling hydrocarbons. As regards the latter item the following may be remarked. If in the two-step process one is prepared to accept separation of steam in the form of water, the division of the reaction product of the second step can be simply brought about by bringing the product to a room temperature, so that the hydrocarbons are divided into substantially C.sub.4.sup.- hydrocarbons which are recycled to the first step on the one hand and C.sub.5.sup.+ hydrocarbons which constitute the end product of the process on the other hand. In such a procedure it is the C.sub.5.sup.+ selectivity of the catalyst in the second step in particular which plays an important role. For at a given activity fewer C.sub.4.sup.- hydrocarbons will be formed according as the catalyst has a higher C.sub.5.sup.+ selectivity, and therefore a smaller recycle stream will be sufficient. However, if in the two-step process it is the object to separate the steam per se, the division of the reaction product of the second step should be carried out at an elevated temperature, notably at a temperature which lies above the dew point of water at the prevailing pressure. In actual practice this means that where the hydrocarbons are concerned, there will be a division into substantially hydrocarbons having at most eight carbon atoms per molecule (hereinafter referred to as "C.sub.8.sup.- hydrocarbons")on the one hand and substantially hydrocarbons having at least nine carbon atoms per molecule (hereinafter referred to as "C.sub.9.sup.+ hydrocarbons") on the other hand. In such a procedure it is the C.sub.9.sup.+ selectivity of the catalyst in the second step in particular that plays an important role. It should be high.
In order to get a fair knowledge of the influence of steam on the performance of Fischer-Tropsch catalysts, an investigation was carried out in which these catalysts were used for the conversion of gas mixtures, some containing steam in addition to H.sub.2 and CO, some not. It was found that the presence of steam in the H.sub.2 /CO mixture decreased the activity of these catalysts. As regards the C.sub.9.sup.+ selectivity the investigation yielded a surprising find. Contrary to other Fischer-Tropsch catalysts upon whose C.sub.9.sup.+ selectivity the presence of steam has no, or else an adverse, effect, it was found for a certain group of cobalt catalysts that the presence of steam led to a considerable increase in their C.sub.9.sup.+ selectivity. The Fischer-Tropsch catalysts displaying this surprising behavior comprise silica, alumina or silica-alumina as carrier material and cobalt together with zirconium, titanium and/or chromium as catalytically active metals, in such quantities that in the catalysts there are present 3-60 pbw cobalt and 0.1-100 pbw zirconium, titanium and/or chromium per 100 pbw carrier material. The catalysts are prepared by depositing the metals involved by kneading and/or impregnation on the carrier material. For further information on the preparation of these catalysts by kneading and/or impregnation reference is made to U.S. Pat. No. 4,499,209, issued Feb. 12, 1985 and which and is incorporated herein by reference.
When a cobalt catalyst belonging to the above-mentioned class is used for the conversion of a H.sub.2 /CO mixture containing no steam, it is seen that under the given reaction conditions this catalyst, in addition to a high stability and C.sub.9.sup.+ selectivity, has a very high activity. If the same catalyst is used under the same reaction conditions for converting a gas mixture containing steam in addition to H.sub.2 and CO, a decrease in activity is seen, as remarked earlier, which decrease is smaller, by the way, than seen for other Fischer-Tropsch catalysts when an equal amount of steam is added to the gas mixture to be converted. However, for the cobalt catalysts there is seen beside the decrease in activity a considerable rise in C.sub.9.sup.+ selectivity. In view of the very high degree of activity of the present cobalt catalysts some loss of activity in exchange for a considerable increase in C.sub.9.sup.+ selectivity is quite acceptable for an operation on a technical scale. These special features combined with a very high H.sub.2 /CO consumption ratio of about 2 render the cobalt catalysts eminently suitable for use in the second step of the afore-mentioned two-step process for the preparation of C.sub.9.sup.+ hydrocarbons which is carried out with recycle of a steam-containing gaseous fraction.